1. Field of the Invention
This invention relates to a grease composition for use in constant velocity joints of vehicles, particularly double-offset type constant velocity joints. More particularly, it relates to a grease composition for constant velocity joint which can efficiently lubricate a portion to be lubricated, effectively reduce wearing, control vibrations and better improve the durable life, because the constant velocity joint is under severe conditions and is apt to be worn and generate abnormal vibrations and the like.
2. Disclosure of the Related Art
Heretofore, lithium base extreme pressure grease containing sulfur-phosphorus series extreme pressure additive, lithium base extreme pressure grease containing molybdenum disulfide and the like have been used in this type of constant velocity joint. Furthermore, Japanese Patent laid open No. 62-207397 discloses that sulfur-phosphorus base extreme pressure additive comprised of molybdenum sulfide dialkyldithiocarbamate and at least one of sulfurized oil, olefin sulfide, tricresyl phosphate, trialkylthio phosphate and zinc dialkyldithio phosphate is suitable as an essential component in the extreme pressure grease, but is not said to be sufficient and is found wanting from a viewpoint of noise reduction and durability.
Since the use of front-engine, front-wheel drive vehicles as well as functional 4-wheel drive vehicles is rapidly increasing from a viewpoint of weight reduction, safety of living space and the like, constant velocity joints (CVJ) are widely used in these vehicles. In FIG. 1 is shown a double offset type joint (DOJ) used as a slide type plunging joint among these constant velocity joints. When the joint transmits a rotating torque at a state of taking an operating angle in the double offset type joint, complicated rolling and sliding motions are created in the fitting of a ball 5 between a track groove 3 of an outer member 1 and a track groove 4 of an inner member 2 and hence force is generated in an axial direction of the joint through a friction resistance of a sliding portion. Such a force is called an induced thrust. Moreover, six track grooves 3 are arranged at an interval of 60.degree. in the inner surface of the outer member 1 in the double offset type joint, so that six induced thrusts are generated per one rotation of the joint.
When the generation cycle of the induced thrust matches with natural frequencies of engine, vehicle body, suspension and the like, resonance is induced in the vehicle body to give an uncomfortability to crews, so that it is desired to reduce the induced thrust as far as possible. Further, when the vehicle is actually run at a high speed, there exists the inconvenience of generating beat noise, muddy noise or the like. Moreover, the lubricating conditions in the double offset type joint becomes more severe with the weight reduction and high output power of the vehicle, and hence it is required to prevent surface peeling (flaking) at friction surface due to metal fatigue or to improve the durability of the joint against damage or the like.
The known solutions to these problems, including the conventional lithium base extreme pressure grease containing sulfur-phosphorus series extreme pressure additive and lithium base extreme pressure grease containing molybdenum disulfide, still have a problem in resisting vibration and are not satisfactory from the standpoint of durability because the wearing is substantial under a high contact pressure and the flaking resistance is insufficient. On the other hand, the grease described in Japanese Patent laid open No. 62-207397 is insufficient to reduce generated vibrations and to resist flaking.
As a grease used under lubricating conditions is easily apt to cause the wearing and to generate vibrations, greases having a lower friction coefficient and an excellent flaking resistance are suitable since there is a known interrelation between friction coefficient and induced thrust in the resistance to vibrations.
As an evaluation of vibration resistance, the induced thrust in the actual joint was measured, and in addition, the friction coefficient, which interrelates the induced thrust of the actual joint was measured by means of a Savan's friction and wear testing machine. Furthermore, the flaking resistance was evaluated as a durability by a table test using the actual joint. As a result, the inventors have found that the combined effect of friction coefficient reduction and flaking life increase can be obtained by a combination of (A) molybdenum sulfide dialkyldithiocarbamate, (B) molybdenum disulfide, (C) zinc dithiophosphate and (D) an oiliness agent composed of one or more of vegetable oils and fats, and the invention has been accomplished.